Allocation of cache storage among applications based on application priority and minimum retention time for tracks in least recently used demoting schemes

ABSTRACT

A computational device receives an indication of minimum retention times in a cache for a plurality of tracks for applications. In response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, the maximum percentage of cache for allocation to the first type of application is set to a default value. In response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, an entirety of the cache or a percentage of the cache that is greater than the default value is allocated for the second type of application. A least recently used based replacement of tracks is performed in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated.

BACKGROUND 1. Field

Embodiments relate to the allocation of cache storage among applications based on application priority and minimum retention time for tracks in least recently used demoting schemes.

2. Background

In certain storage system environments, a storage controller (or a storage controller complex) may comprise a plurality of storage servers that are coupled to each other. The storage controller allows host computing systems to perform input/output (I/O) operations with storage devices controlled by the storage controller, where the host computing systems may be referred to as hosts.

The storage controller may include two or more servers, where each server may be referred to as a node, a storage server, a processor complex, a Central Processor Complex (CPC), or a Central Electronics Complex (CEC). Each server may have a plurality of processor cores and the servers may share the workload of the storage controller. In a two server configuration of the storage controller that is also referred to as a dual server based storage controller, in the event of a failure of one of the two servers, the other server that has not failed may take over the operations performed by the failed server.

Data written from a host may be stored in the cache of the storage controller, and at an opportune time the data stored in the cache may be destaged (i.e., moved or copied) to a storage device. Data may also be staged (i.e., moved or copied) from a storage device to the cache of the storage controller. The storage controller may respond to a read I/O request from the host from the cache, if the data for the read I/O request is available in the cache, otherwise the data may be staged from a storage device to the cache for responding to the read I/O request. A write I/O request from the host causes the data corresponding to the write to be written to the cache, and then at an opportune time the written data may be destaged from the cache to a storage device. Since the storage capacity of the cache is relatively small in comparison to the storage capacity of the storage devices, data may be periodically destaged from the cache to create empty storage space in the cache. Data may be written and read from the cache much faster in comparison to reading and writing data from a storage device. In computing, cache replacement policies are used to determine which items to discard (i.e., demote) from the cache to make room for new items in the cache. In a least recently used (LRU) cache replacement policy, the least recently used items are discarded first.

U.S. Pat. No. 9,460,025 at least discusses methods for minimizing contention among multiple threads include maintaining a plurality of linked lists of elements. Australia Patent No. AU2015201273 at least discusses a system and a method in which a currently requested item of information is stored in a cache based on whether it has been previously requested and, if so, the time of the previous request. U.S. Pat. No. 9,734,066 at least discusses determining a workload level associated with an expandable data buffer, where the expandable data buffer and an expandable mapping table cache are stored in internal memory and the expandable mapping table cache is used to store a portion of a mapping table that is stored on external storage. U.S. Pat. No. 8,838,903 at least discusses a hierarchical data-storage system having a volatile storage medium, a first non-volatile storage medium, and a controller including a ranking engine tracking data writes to each of the memory mediums. U.S. Pat. No. 9,158,671 at least discusses a memory system that includes a nonvolatile memory having a main region and a cache region; and a memory controller having migration manager managing a migration operation that moves data from cache region to the main region by referencing a Most Recently Used/Least Recently Used (MRU/LRU) list. U.S. Pat. No. 9,189,423 at least discusses a method and apparatus for controlling a cache. U.S. Pat. No. 6,745,295 at least discusses a system, computer program product and method for reconfiguring a cache.

SUMMARY OF THE PREFERRED EMBODIMENTS

Provided are a method, system, and computer program product in which a computational device receives an indication of minimum retention times in a cache for a plurality of tracks of a plurality of applications. In response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, the computational device sets the maximum percentage of cache for allocation to the first type of application to a default value. In response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, the computational device allows an entirety of the cache or a higher percentage of the cache that is greater than the default value to be allocated for the second type of application. A least recently used (LRU) based replacement of tracks is performed in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated for the plurality of applications. As a result, the storage of the cache is distributed fairly among applications that request minimum retention time for tracks in the cache while prioritizing certain applications.

In certain embodiments, the second type of application includes at least one of an operating system, a device driver, and a real-time application, wherein the second type of application is not throttled by demoting tracks of the second type of application prior to expiry of minimum retention time of the tracks of the second type of application, even if the tracks of the second type of application occupy the entirety of the cache. As a result, high priority applications are allowed to use the entirety of the cache or a higher percentage of the cache that is greater than the default value.

In further embodiments, the default value for the first type of application does not exceed 25% of the cache. As a result, the minimum retention time tracks of this type of application may not occupy more than a predetermined percentage (e.g., 25%) of the cache.

In additional embodiments, in response to determining that an application has specified a maximum percentage of cache for allocation to application, the computational device sets the maximum percentage of cache for allocation to the application to the specified maximum percentage. As a result, the requirements indicated by the application on the maximum percentage of cache to allocate for the application are satisfied whenever that is possible.

In further embodiments, in response to determining that tracks of an application that are stored in the cache exceed the maximum percentage of cache allocated for the application, the computational device demotes one or more tracks of the application from the cache even though a minimum retention time in cache has been indicated for the one or more tracks of the application, while performing least recently used (LRU) based replacement of tracks in the cache. As a result, existing LRU based demotion from the cache continue to occur.

In certain embodiments, the plurality of tracks is a first plurality of tracks, and wherein no indication of a minimum retention time in the cache is received for a second plurality of tracks. As a result, existing LRU based demotion continue to occur for tracks that have no minimum retention time,

In further embodiments, the computational device demotes a track of the first plurality of tracks from the cache, in response to determining that the track is a LRU track in a LRU list of tracks in the cache and the track has been in the cache for a time that exceeds the minimum retention time. The computational device demotes a track of the second plurality of tracks from the cache, in response to determining that the track of the second plurality of tracks is a LRU track in a LRU list. As a result both minimum retention time tracks and tracks with no minimum retention time tracks are processed in a LRU based demotion mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers represent corresponding parts throughout:

FIG. 1 illustrates a block diagram of a computing environment comprising a storage controller coupled to one or more hosts and one or more storage devices, in accordance with certain embodiments;

FIG. 2 illustrates a block diagram that shows demotion of tracks from an LRU list that does not incorporate a minimum retention time for selected tracks or enforce limits on the number of minimum retention time tracks in cache for an application, in accordance with certain embodiments;

FIG. 3 illustrates a block diagram that shows the eligibility for demotion of different types of tracks and threshold for minimum retention time tracks for different types of applications in cache, in accordance with certain embodiments;

FIG. 4 illustrates a flowchart that shows the determination of the maximum percentage of cache that is to be allocated for an application, in accordance with certain embodiments;

FIG. 5 illustrates a flowchart that shows the addition of a track with a minimum retention time to the cache where the request for addition is made by an application having an application identifier, in accordance with certain embodiments;

FIG. 6 illustrates a flowchart that shows the demote process for a track as performed by a cache management application, in accordance with certain embodiments;

FIG. 7 illustrates a block diagram that shows predetermined conditions based on which a demotion decision for a track with minimum retention time is made when limits are placed on the number of minimum retention time tracks for applications, in accordance with certain embodiments;

FIG. 8 illustrates a flowchart that shows how tracks are demoted by a cache management application that incorporates minimum retention time of tracks with least recently used (LRU) based cache replacement policies for different types of applications, where the amount of minimum retention time tracks that an application may store at a time in the cache may be limited, in accordance with certain embodiments;

FIG. 9 illustrates a block diagram of a cloud computing environment, in accordance with certain embodiments;

FIG. 10 illustrates a block diagram of further details of the cloud computing environment of FIG. 9, in accordance with certain embodiments; and

FIG. 11 illustrates a block diagram of a computational system that shows certain elements that may be included in the storage controller or the host, as described in FIGS. 1-10, in accordance with certain embodiments.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying drawings which form a part hereof and which illustrate several embodiments. It is understood that other embodiments may be utilized and structural and operational changes may be made.

A cache replacement policy in a storage controller may be LRU based. In a conventional LRU based mechanism, a track upon being accessed is added to a most recently used (MRU) end of a LRU list. As the track ages (i.e., is not used) the track moves to the LRU end of the LRU list and then gets demoted (i.e., discarded or removed) from the cache.

In certain embodiments, an application may provide indications to a storage controller to store tracks used by the application for a minimum period of time in the cache, where the minimum period of time is referred to as a minimum retention time. The application is likely to reaccess (i.e., access once again) the track within the minimum retention time, so it is desirable not to demote the track from the cache until the expiry of the minimum retention time. However, if the minimum retention time is strictly adhered to for all applications then cache may become full with such minimum retention tracks. As a result, the performance of applications that use the cache may be impacted.

If the minimum retention time is not specified by the application then the cache management application uses the conventional LRU based mechanism to age out the track. However, if the minimum retention time is strictly satisfied for all applications, then the cache may become full of tracks with minimum retention time and it is possible that no tracks may be demoted. Certain embodiments handle such situations while attempting to adhere to minimum time requirements fur caching certain tracks. In such embodiments, if a track with minimum retention time is accessed and the track moves to the LRU end of the LRU list before the minimum retention time is exceeded then instead of demoting the track, the track may be moved to the MRU end of the LRU list. If the cache is full of tracks with minimum retention time, then some of the tracks with minimum retention time are demoted by not strictly satisfying the minimum retention time requirements. In order to ensure fairness among various applications that indicate minimum retention time for tracks, if an application has minimum retention time tracks beyond a certain percentage of the cache then tracks for that application are demoted even when the minimum retention time for the tracks have not been exceeded.

In certain embodiments, certain applications (referred to as high priority applications), such as, those related to the operating system and certain device drivers, are provided priority over other applications when a plurality of applications are allowed to provide application hints on a minimum time to cache. As a result, hints provided by high priority applications (e.g., those related to operating system and certain device drivers) are provided greater weightage over hints provided by low priority applications, while at the same time ensuring some degree of fairness in accommodating the hints provided by the low priority applications.

In certain embodiments in which an application does not specify the maximum percentage of cache to use for minimum retention time tracks, a default amount (e.g. 10%) of the cache is allocated for the application if the application is not a high priority application. If the application is a high priority application (e.g., an operating system process that must have real-time performance) the application is not throttled, i.e. the application is allowed to use any amount of cache.

Certain embodiments provide improvements to computer technology, by integrating application indicated minimum retention time for tracks to LRU based track demoting schemes in a cache management system of a storage controller while ensuring fairness in cache allocation among a plurality of applications by restricting the amount of minimum retention time tracks each application may have in the cache while integrating the priority of applications. All applications that indicate minimum retention time for tracks share the cache equitably while providing preference to high priority applications such as operating system processes that may need to provide real-time performance. As a result, caching operations in a storage controller take account of hints (i.e., indications) provided by an application to retain certain tracks in cache for at least a minimum amount of time whenever that is feasible while at the same time allocating the cache fairly among applications that request minimum retention time tracks and providing preference to high priority applications.

Exemplary Embodiments

FIG. 1 illustrates a block diagram of a computing environment 100 comprising a storage controller 102 coupled to one or more hosts 104 and one or more storage devices 106, 108, in accordance with certain embodiments.

The storage controller 102 allows the one or more hosts 104 to perform input/output (I/O) operations with logical storage maintained by the storage controller 102. The physical storage corresponding to the logical storage may be found in one or more of the storage devices 106, 108 and/or a cache 110 of the storage controller 102.

The storage controller 102 and the hosts 104 may comprise any suitable computational device including those presently known in the art, such as, a personal computer, a workstation, a server, a mainframe, a hand held computer, a palm top computer, a telephony device, a network appliance, a blade computer, a processing device, a controller, etc. In certain embodiments, the storage controller 102 may be comprised of a plurality of servers. The plurality of servers may provide redundancy because if one server undergoes a failure from which recovery is not possible, an alternate server may perform the functions of the server that failed. Each of the plurality of servers may be referred to as a processing complex and may include one or more processors and/or processor cores.

The storage controller 102 and the one or more hosts 104 may be elements in any suitable network, such as, a storage area network, a wide area network, the Internet, an intranet. In certain embodiments, storage controller 102 and the one or more hosts 104 may be elements in a cloud computing environment.

The cache 110 may be any suitable cache known in the art or developed in the future. In some embodiments, the cache 110 may be implemented with a volatile memory and/or non-volatile memory. The cache 110 may store both modified and unmodified data, where a cache management application 112 may periodically demote (i.e., move) data from the cache 110 to storage devices 106, 108 controlled by the storage controller 102. In certain embodiments, cache management application 112 may be implemented in software, firmware, hardware or any combination thereof.

The plurality of storage devices 106, 108 may be comprised of any storage devices known in the art. For example, the storage device 106 may be a solid state drive (SSD) and the storage device 108 may be a hard disk drive (HDD).

A LRU list 114 is maintained in the storage controller 102 by the cache management application 112. The cache management application 112 receives indications from one or more host applications on whether tracks used by the host application 116 should be protected from demotion from the cache 110 for a time duration referred to as a minimum retention time 118. A plurality of tracks 120, 122 may have minimum retention times 124, 126 indicated by host applications. Another plurality of tracks 128, 130 may have no indications of minimum retention time (as shown via reference numerals 132, 134). The LRU list 114 may include some of the plurality of tracks 120, 122 and some of the plurality of tracks 128, 130, i.e., the LRU list 114 includes tracks in the cache 110 with minimum retention time and tracks in the cache 110 without minimum retention time. An indicator 136 maintains the number of tracks with minimum retention time in the cache 110. It should be noted that each track stored in the cache 110 is indicated in the LRU list 114, and the list 114 is used by the cache management application 112 to determine which tracks to demote from the cache 110.

The storage controller 102 also includes an indicator, such as a clock, that provides the current time 138 of the storage controller 102, where the current time 138 and a timestamp associated with a track when the track is staged to the cache 110 are used to determine the amount of time the track has been stored in the cache 110.

For each of a plurality of host applications 116 that requests minimum retention time tracks in the cache 110, the cache management application 112 stores an application identifier that corresponds to the host application. A plurality of application identifiers 140, 142 are shown in FIG. 1, where each application identifier is a unique identification corresponding to a host application (i.e., an application identifier uniquely identifies a host application). For each application identifier an indication may be stored of a maximum cache allocation 144, 146 that is allowed for the corresponding host application. The maximum cache allocation 144, 146 may be a default value (e.g., 25% of the cache) for certain applications that do not request a maximum cache allocation. The maximum cache allocation 144, 146 may be the entirety (i.e., 100%) of the cache for certain high priority applications (such as operating system processes with real-time processing requirements). For each application identifier another indication may be stored of a number of minimum retention time tracks allocated (reference numerals 148, 150) in cache for the host application. For example, for application identifier 140, the maximum cache allocation 144 may be 10000 tracks (e.g., 10% of the cache 110 if the cache has 100000 tracks), and the number of minimum retention time tracks allocated at 3 PM may be 7000 tracks. In such embodiments, the cache management application 112 may allocate a maximum of 3000 more minimum retention time tracks for the host application in the cache 110.

For each application identifier 140, 142, an application priority 152, 154 may be maintained. The application priority 152, 154 may indicate whether an application is a high priority application or not. High priority applications are allowed to be allocated the entirety of the cache in certain embodiments.

In certain embodiments, the cache management application 112 demotes tracks from the cache 110 by taking into account the LRU list 114 which is augmented with the minimum retention time for tracks where such minimum retention times are available while at the same time ensuring that no host application has minimum retention time tracks allocated in excess of the maximum cache allocation for the host application. It should be noted that high priority applications may have the entirety (i.e., 100%) of the cache allocated. An attempt is made by the cache management application 112 to satisfy the minimum retention time requirements to the extent possible while ensuring fairness in cache allocation among a plurality of host applications, while at the same time preventing the cache 110 from becoming full. While performing such processes, priority is provided to high priority applications by allowing the high priority applications to use the entirety of the cache. A value for a default cache allocation 156 may also be maintained in the storage controller 102 for applications that do not indicate how much of the cache to allocate for minimum retention time tracks.

FIG. 2 illustrates a block diagram 200 that shows demotion of tracks from a LRU list that does not incorporate a minimum retention time for selected tracks or enforce limits on the number of minimum retention time tracks in cache for an application, in accordance with certain embodiments.

For simplicity, only four tracks, denoted as track K 202, track C 204, track D 206, and track B 208 are shown in FIG. 2, although a typical LRU list may have thousands or tens of thousands of tracks.

The LRU end of the list is towards the top (as shown via reference numeral 210). As a result, track K 202 is the least recently used track, and track B 208 is the most recently used track.

Unless minimum retention time information was included with the tracks or the number of minimum retention time tracks for an application was restricted, track K 202 is demoted in a conventional LRU based cache replacement policy (as shown via reference numeral 212). However if track K 202 is likely to seek entry to the cache shortly after demotion (for example in a situation where a host application 116 indicates that track K 202 should have a minimum retention time), then the cache hit ratio would be decreased.

FIG. 3 illustrates a block diagram 300 that shows the eligibility for demotion of different types of tracks in a cache 110 and a threshold for minimum retention time tracks for different types in cache 110, in accordance with certain embodiments.

An application identifier A 302 identifies a host application, and a maximum percentage of cache that can be allocated for tracks with minimum retention time for application identifier A 302 is 10% (as shown via reference numeral 304). An application identifier N 306 identifies another host application, and a maximum percentage of cache that can be allocated for tracks with minimum retention time for application identifier N 306 is 12% (as shown via reference numeral 308).

An application identifier P 310 identifies a host application, and a maximum percentage of cache that can be allocated for tracks with minimum retention time for application identifier A 310 is a default value of 5% (as shown via reference numeral 312). Such default values may be allocated for a host application that does not indicate the maximum percentage of the cache that may be allocated to the host application, where the host application is not a high priority application.

An application identifier Q 314 identifies an application (e.g., a host application or a storage controller application) that is a high priority application, and a maximum percentage of cache that can be allocated for tracks with minimum retention time for application identifier Q 314 is 100% (as shown via reference numeral 316 which indicates that “any amount of cache can be allocated”). Therefore any amount of cache can be allocated for high priority applications such as certain operating system processes, device drivers, or real-time applications.

As shown in FIG. 3, different application identifiers may have a different maximum percentage of cache that can be allocated for the application identifier. The maximum percentage of cache that can be allocated corresponds to the maximum cache allocation 144, 146 indicated in FIG. 1.

There are many tracks that are used by a plurality of host applications, and three types of exemplary tracks are shown via reference numerals 318, 320, 322. Track M1 318 has no minimum retention time 324. As a result track M1 318 is always eligible for demotion when track M1 318 reaches the LRU end of the LRU list 114 (as shown via reference numeral 326).

Track M2 320 has a minimum retention time 328 of 10 seconds. As a result track M2 320 is eligible for demotion if track M2 320 is in cache beyond 10 seconds or under certain predetermined conditions when the cache 110 is in danger of getting full or when a host application corresponding to track M2 320 exceeds its maximum cached allocation (as shown via reference numeral 330).

Track M3 322 has a minimum retention time 332 of 15 seconds. As a result track M3 322 is eligible for demotion if track M3 322 is in cache beyond 15 seconds or under certain predetermined conditions when the cache 110 is in danger of getting full or when a host application corresponding to track M3 322 exceeds its maximum cache allocation (as shown via reference numeral 334).

Therefore, FIG. 3 illustrates certain embodiments in which different tracks have different minimum retention times or no retention time, and where different host applications may have different maximum cache allocation for minimum retention time tracks.

FIG. 4 illustrates a flowchart 400 that shows the determination of the maximum percentage of cache that is to be allocated for an application, in accordance with certain embodiments. The operations shown in FIG. 4 may be performed by the cache management application 112 that executes in the storage controller 102.

Control starts at block 402 in which the cache management application 112 determines whether an application specifies the maximum percentage of cache that should be allocated to the application. The application may correspond an application or process that executes in the storage controller.

If the cache management application 112 determines that the application specifies the maximum percentage of cache that should be allocated to the application (“Yes” branch 404) control proceeds to block 406 in which the cache management application 112 indicates the maximum percentage of the cache that can be allocated to the application to correspond to the maximum percentage indicated by the application.

If the cache management application 112 determines that the application has not specified the maximum percentage of cache that should be allocated to the application (“No” branch 408) control proceeds to block 410 in which the cache management application 112 determines whether the application is a critical and/or high priority application. If so (“Yes” branch 412) control proceeds to block 414 in which the cache management application 112 sets the maximum percentage of cache than can be allocated to the application to be 100% or some other percentage greater than a default value (e.g., if the default value is 25% then the maximum percentage of cache than can be allocated to the application is greater than 25%). As a result critical and/or high-priority applications (such as operating system processes, certain real time applications, certain device drivers, etc.) are allowed usage of 100% of the cache or at least allowed usage of a percentage of the cache that is greater than a default value, where the default value is used for applications that do not indicate any percentage of cache to be allocated.

If at block 410 the cache management application 112 determines that the application is not a critical and/or high priority application (“No” branch 416) control proceeds to block 418 in which the cache management application 112 sets the maximum percentage of cache that can be allocated to the application to be a default value (e.g. 10%, 25%, etc.).

Therefore, FIG. 4 illustrates certain embodiments in which different types of applications are allowed to be allocated different percentages of the cache for processing of minimum retention time tracks and other tracks.

FIG. 5 illustrates a flowchart 500 that shows the addition of a track with a minimum retention time to the cache 110 where the request for addition is made by an application having an application identifier, in accordance with certain embodiments. The operations shown in FIG. 5 may be performed by the cache management application 112 that executes in the storage controller 102. The operations shown in FIG. 5 improves the performance of the storage controller 102 by preventing potential cache misses should there be indications of minimum retention time 118 for certain tracks in the cache 110 while at the same time allocating the cache fairly among all applications that have minimum retention time tracks.

Control starts at block 502 in which the cache management application 112 adds the track with minimum retention time to the MRU end of the LRU list 114. Control proceeds to block 504 in which the cache management application 112 places a timestamp to indicate when the track was added to the MRU end of the LRU list 114. The timestamp is kept in association with the track that is added to the LRU list 114

From block 504 control proceeds to block 506 in which the cache management application 112 stores the minimum retention time of the track in association with the track that is added to the LRU list 114. The cache management application 112 then increments (i.e., adds the number 1) to the number of minimum retention time tracks 136 (as shown via block 508).

From block 508 control proceeds to block 510 in which the cache management application 112 increments the number of “minimum retention time tracks allocated” 148, 150 for the track corresponding to the application that requested the minimum retention time track.

Therefore, FIG. 5 shows certain embodiments in which tracks are added to the LRU list 114 with indication of the minimum retention time and a timestamp that records the time at which the track was added to the LRU list. The number of tracks with minimum retention time is updated when a track with a minimum retention time is added to the LRU list 114. It should be noted that the LRU list 114 is updated each time a track is added to the cache 110. The number of minimum retention time tracks allocated to each application identifier is also updated when a track is added to the LRU list 114.

FIG. 6 illustrates a flowchart 600 that shows the demote process for a track as performed by a cache management application 112, in accordance with certain embodiment. The operations shown in FIG. 6 improves the performance of the storage controller 102 by reducing cache misses should there be indications of minimum retention time 118 for certain tracks in the cache 110.

Control starts at block 602 in which the cache management application 112 starts evaluating the track at the LRU end of the LRU list 114 for potential demotion from the cache 110. Control proceeds to block 604 in which the cache management application 112 determines whether the track has a minimum retention time. If not (“No” branch 606), then the cache management application 112 demotes (at block 608) the track from the cache 110.

If at block 604 the cache management application 112 determines that the track has a minimum retention time (“Yes” branch 610) then control proceeds to block 612 in which the cache management application 112 determines whether the difference of the current time 138 from the timestamp of the track is greater than the minimum retention time of the track (i.e., whether the track has been in the cache for a time that exceeds the minimum retention time). If so (“Yes” branch 614) then the track is demoted (at block 616) and the cache management application 112 decrements (i.e., subtracts the number 1) the number of minimum retention time tracks 136 in the cache 110 (at block 618). The cache management application 112 also decrements (i.e., reduces by 1) the number of minimum retention time tracks allocated for the application (as designated by the application identifier) corresponding to the demoted track (at block 630). Therefore, the number of minimum retention time tracks of the application that have been allocated in the cache is decremented on demotion of a minimum retention time track of the application.

If at block 612 it is determined that the difference of the current time 138 from the timestamp of the track is not greater than the minimum retention time of the track (i.e., the track has not been in the cache for a time that exceeds the minimum retention time) [“No” branch 620] control proceeds to block 622 to determine whether the track should be demoted to prevent the cache 110 from becoming full.

At block 622 the cache management application 112 determines whether there are too many tracks with minimum retention time or too few tracks that are eligible for demotion from the cache that may cause the cache to become full or whether any application has too many minimum retention time tracks. What is “too many” and what is “too few” is determined based on certain predetermined conditions (i.e., predetermined parameters that may be assigned to predetermined values), where some exemplary predetermined conditions are described later in FIG. 7.

If at block 622 the cache management application 112 determines that there are too many tracks with minimum retention time or too few tracks that are eligible for demotion from the cache that may cause the cache to become full or an application has too many minimum retention time tracks (“Yes” branch 624) then control proceeds to block 616 in which the track is demoted and the number of minimum retention time tracks is decremented (at block 618). The cache management application 112 also decrements (i.e., reduces by 1) the number of minimum retention time tracks allocated for the application (as designated by the application identifier) corresponding to the demoted track (at block 630). Therefore, the number of minimum retention time tracks of the application that have been allocated in the cache decremented on demotion of a minimum retention time track of the application.

If at block 622 the cache management application 112 determines that that neither are there too many tracks with minimum retention time nor are there too few tracks that are eligible for demotion from the cache that may cause the cache to become full, nor are there any application that has too many minimum retention time tracks (“No” branch 626) then there is no danger of the cache 110 becoming full or an application securing too much cache space, and the cache management application 112 moves (at block 628) the track to the MRU end of the LRU list 114 (i.e., the track is retained in the cache 110 and moved to the MRU end of the LRU list 114).

Therefore, FIG. 6 illustrates certain embodiments in which tracks that are indicated as having a minimum retention time arc preferred for storing in the cache at least till the expiry of the minimum retention time unless the cache 110 is in danger of becoming full or an application for the track has too many minimum retention time tracks based on the maximum allowed percentage of cache that is allowed to be allocated to the application

FIG. 7 illustrates a block diagram 700 that shows predetermined conditions based on which a demotion decision for a track with minimum retention time is made, in accordance with certain embodiments.

A threshold indicating the percentage of tracks with minimum retention time above which tracks with minimum retention time are eligible for demotion is maintained (as shown via reference numeral 702) in the storage controller 102. For example in certain embodiments the threshold may be 50%, and as a result more if more than 50% of the tracks in the cache are tracks with minimum retention time, then one or more tracks with minimum retention time may be demoted based on the determination in block 622 of FIG. 6.

A “maximum number” of tracks that are eligible for demotion in a “predetermined number” of tracks towards the LRU end of the LRU list 114 is maintained (as shown via reference numeral 704) in the storage controller 102. For example, if the cache management application 112 scans N tracks from the bottom of the LRU (i.e., the N least recently used tracks, where N is a number) and finds less than M tracks eligible to demote because of minimum retention time requirements, then tracks with a minimum retention time may be demoted (i.e., M is the maximum number and N is the “predetermined number”). For example, if the cache management application 112 scans 1000 tracks from the bottom of LRU (i.e., the 1000 least recently used tracks) to demote and only finds less than 100 as being eligible for demotion then tracks with minimum retention time may be demoted based on the determination in block 622 of FIG. 6.

Another predetermined condition is that an application corresponding to a minimum retention time track has a greater than a predetermined percentage (e.g. 10%) of the cache filled with minimum retention time tracks. Such minimum retention time tracks are demoted whenever possible, to ensure fairness in cache allocation among a plurality of applications. It should be noted that certain critical or high priority applications are allowed to use 100% of the cache.

FIG. 8 illustrates a flowchart 800 that shows how tracks are demoted by a cache management application 112 that incorporates minimum retention time of tracks with least recently used (LRU) based cache replacement policies for different types of applications, where the amount of minimum retention time tracks that an application may store at a time in the cache may be limited, in accordance with certain embodiments. The operations shown in FIG. 8 may be performed by the cache management application 112 that executes in the storage controller 102. The operations shown in FIG. 8 improves the performance of the storage controller 102 by preventing potential cache misses should there be indications of minimum retention time 118 for certain tracks in the cache 110 and also ensures fairness in cache allocation among applications while taking into account the priority of applications.

Control starts at block 802 in which a computational device 102 receives an indication of a minimum retention time in a cache 110 for a first plurality of tracks of an application 116. No indication of a minimum retention time is received for a second plurality of tracks. From block 802 control may proceed in parallel to blocks 804, 806, 808.

At block 804, in response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, the cache management application 112 sets the maximum percentage of cache for allocation to the first type of application to a default value (e.g., 25% of the cache).

At block 806, in response to determining that a second type of application (e.g., a high priority application) has not specified a maximum percentage of cache for allocation to the second type of application, the cache management application 112 allows an entirety of the cache or a higher percentage of the cache that is greater than the default value to be allocated for the second type of application.

At block 808, in response to determining that an application has specified a maximum percentage of cache for allocation to application, the cache management application 112 sets the maximum percentage of cache for allocation to the application to the specified maximum percentage.

From block 804, 806, 808 control proceeds to block 810 in which the cache management application 112 performs least recently used (LRU) based replacement of tracks in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that is allocated for a plurality of applications.

Therefore, FIGS. 1-8 illustrate certain embodiments in which hints from host applications may indicate a minimum retention time in cache for certain tracks used by the host applications. The storage controller integrates the minimum retention time into LRU based cache replacement policies while ensuring that limits are placed on the ability of each host application to use more than a certain percentage of the cache for storage of minimum retention time tracks. Minimum retention time requirements are satisfied whenever that is feasible without filling up the cache while at the same time distributing the storage space of the cache in a fair manner among the plurality of applications, while at the same time allowing high priority application to use the entirety of the cache.

Cloud Computing Environment

Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.

Referring now to FIG. 9 an illustrative cloud computing environment 50 is depicted. As shown, cloud computing environment 50 comprises one or more cloud computing nodes 10 with which local computing devices used by cloud consumers, such as, for example, personal digital assistant (PDA) or cellular telephone 54A, desktop computer 54B, laptop computer 54C, and/or automobile computer system 54N may communicate. Nodes 10 may communicate with one another. They may be grouped (not shown) physically or virtually, in one or more networks, such as Private, Community, Public, or Hybrid clouds as described hereinabove, or a combination thereof. This allows cloud computing environment 50 to offer infrastructure, platforms and/or software as services for which a cloud consumer does not need to maintain resources on a local computing device. It is understood that the types of computing devices 54A-N shown in FIG. 9 are intended to be illustrative only and that computing nodes 10 and cloud computing environment 50 can communicate with any type of computerized device over any type of network and/or network addressable connection (e.g., using a web browser).

Referring now to FIG. 10, a set of functional abstraction layers provided by cloud computing environment 50 (FIG. 9) is shown. It should be understood in advance that the components, layers, and functions shown in FIG. 10 are intended to be illustrative only and embodiments of the invention are not limited thereto.

Hardware and software layer 60 includes hardware and software components, Examples of hardware components include mainframes, in one example IBM zSeries* systems; RISC (Reduced Instruction Set Computer) architecture based servers, in one example IBM pSeries* systems; IBM xSeries* systems; IBM BladeCenter* systems; storage devices; networks and networking components. Examples of software components include network application server software, in one example IBM WebSphere* application server software; and database software, in one example IBM DB2* database software. *IBM, zSeries, pSeries, xSeries, BladeCenter, WebSphere, and DB2 are trademarks of International Business Machines Corporation registered in many jurisdictions worldwide.

Virtualization layer 62 provides an abstraction layer from which the following examples of virtual entities may be provided: virtual servers; virtual storage; virtual networks, including virtual private networks; virtual applications and operating systems; and virtual clients.

In one example, management layer 64 may provide the functions described below. Resource provisioning provides dynamic procurement of computing resources and other resources that arc utilized to perform tasks within the cloud computing environment. Metering and Pricing provide cost tracking as resources are utilized within the cloud computing environment, and billing or invoicing for consumption of these resources. In one example, these resources may comprise application software licenses. Security provides identity verification for cloud consumers and tasks, as well as protection for data and other resources. User portal provides access to the cloud computing environment for consumers and system administrators. Service level management provides cloud computing resource allocation and management such that required service levels are met. Service Level Agreement (SLA) planning and fulfillment provide pre-arrangement for, and procurement of, cloud computing resources for which a future requirement is anticipated in accordance with an SLA.

Workloads layer 66 provides examples of functionality for which the cloud computing environment may be utilized. Examples of workloads and functions which may be provided from this layer include: mapping and navigation; software development and lifecycle management; virtual classroom education delivery; data analytics processing; transaction processing; and cache management mechanism 68 as shown in FIGS. 1-10.

Additional Embodiment Details

The described operations may be implemented as a method, apparatus or computer program product using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. Accordingly, aspects of the embodiments may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the embodiments may take the form of a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present embodiments.

The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A non-exhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punch-cards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiber-optic cable), or electrical signals transmitted through a wire.

Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.

Computer readable program instructions for carrying out operations of the present embodiments may be assembler instructions, instruction-set-architecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, state-setting data, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++ or the like, and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present embodiments.

Aspects of the present embodiments are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.

The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instruction.

FIG. 11 illustrates a block diagram that shows certain elements that may be included in the storage controller 102 or the host 104, or other computational devices in accordance with certain embodiments. The system 1100 may include a circuitry 1102 that may in certain embodiments include at least a processor 1104. The system 1100 may also include a memory 1106 (e.g., a volatile memory device), and storage 1108. The storage 1108 may include a non-volatile memory device (e.g., EEPROM, ROM, PROM, flash, firmware, programmable logic, etc.), magnetic disk drive, optical disk drive, tape drive, etc. The storage 1108 may comprise an internal storage device, an attached storage device and/or a network accessible storage device. The system 1100 may include a program logic 1110 including code 1112 that may be loaded into the memory 1106 and executed by the processor 1104 or circuitry 1102. In certain embodiments, the program logic 1110 including code 1112 may be stored in the storage 1108. In certain other embodiments, the program logic 1110 may be implemented in the circuitry 1102. One or more of the components in the system 1100 may communicate via a bus or via other coupling or connection 1114. Therefore, while FIG. 11 shows the program logic 1110 separately from the other elements, the program logic 1110 may be implemented in the memory 1106 and/or the circuitry 1102.

Certain embodiments may be directed to a method for deploying computing instruction by a person or automated processing integrating computer-readable code into a computing system, wherein the code in combination with the computing system is enabled to perform the operations of the described embodiments.

The terms “an embodiment”, “embodiment”, “embodiments”, “the embodiment”, “the embodiments”, “one or more embodiments”, “some embodiments”, and “one embodiment” mean “one or more (but not all) embodiments of the present invention(s)” unless expressly specified otherwise.

The terms “including”, “comprising”, “having” and variations thereof mean “including but not limited to”, unless expressly specified otherwise.

The enumerated listing of items does not imply that any or all of the items are mutually exclusive, unless expressly specified otherwise.

The terms “a”, “an” and “the” mean “one or more”, unless expressly specified otherwise.

Devices that are in communication with each other need not be in continuous communication with each other, unless expressly specified otherwise. In addition, devices that are in communication with each other may communicate directly or indirectly through one or more intermediaries.

A description of an embodiment with several components in communication with each other does not imply that all such components are required. On the contrary a variety of optional components are described to illustrate the wide variety of possible embodiments of the present invention.

Further, although process steps, method steps, algorithms or the like may be described in a sequential order, such processes, methods and algorithms may be configured to work in alternate orders. In other words, any sequence or order of steps that may be described does not necessarily indicate a requirement that the steps be performed in that order. The steps of processes described herein may be performed in any order practical. Further, some steps may be performed simultaneously.

When a single device or article is described herein, it will be readily apparent that more than one device/article (whether or not they cooperate) may be used in place of a single device/article. Similarly, where more than one device or article is described herein (whether or not they cooperate), it will be readily apparent that a single device/article may be used in place of the more than one device or article or a different number of devices/articles may be used instead of the shown number of devices or programs. The functionality and/or the features of a device may be alternatively embodied by one or more other devices which are not explicitly described as having such functionality/features. Thus, other embodiments of the present invention need not include the device itself.

At least certain operations that may have been illustrated in the figures show certain events occurring in a certain order. In alternative embodiments, certain operations may be performed in a different order, modified or removed. Moreover, steps may be added to the above described logic and still conform to the described embodiments. Further, operations described herein may occur sequentially or certain operations may be processed in parallel. Yet further, operations may be performed by a single processing unit or by distributed processing units.

The foregoing description of various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended. 

What is claimed is
 1. A method, comprising: receiving, by a computational device, an indication of minimum retention times in a cache for a plurality of tracks of a plurality of applications; in response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, setting the maximum percentage of cache for allocation to the first type of application to a default value; and in response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, allowing an entirety of the cache or a higher percentage of the cache that is greater than the default value to be allocated for the second type of application; and performing least recently used (LRU) based replacement of tracks in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated for the plurality of applications.
 2. The method of claim 1, wherein the second type of application includes at least one of an operating system, a device driver, and a real-time application, and wherein the second type of application is not throttled by demoting tracks of the second type of application prior to expiry of minimum retention time of the tracks of the second type of application, even if the tracks of the second type of application occupy the entirety of the cache.
 3. The method of claim 1, wherein the default value for the first type of application does not exceed 25% of the cache.
 4. The method of claim 1, the method further comprising: in response to determining that an application has specified a maximum percentage of cache for allocation to application, setting the maximum percentage of cache for allocation to the application to the specified maximum percentage.
 5. The method of claim 1, the method further comprising: in response to determining that tracks of an application that are stored in the cache exceed the maximum percentage of cache allocated for the application, demoting, by the computational device, one or more tracks of the application from the cache even though a minimum retention time in cache has been indicated for the one or more tracks of the application, while performing least recently used (LRU) based replacement of tracks in the cache.
 6. The method of claim 1, wherein the plurality of tracks is a first plurality of tracks, and wherein no indication of a minimum retention time in the cache is received for a second plurality of tracks.
 7. The method of claim 6, the method further comprising: demoting a track of the first plurality of tracks from the cache, in response to determining that the track is a LRU track in a LRU list of tracks in the cache and the track has been in the cache for a time that exceeds the minimum retention time; and demoting a track of the second plurality of tracks from the cache, in response to determining that the track of the second plurality of tracks is a LRU track in a LRU list.
 8. A system, comprising: a memory; and a processor coupled to the memory, wherein the processor performs operations, the operations performed by the processor comprising: receiving an indication of minimum retention times in a cache for a plurality of tracks of a plurality of applications; in response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, setting the maximum percentage of cache for allocation to the first type of application to a default value; and in response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, allowing an entirety of the cache to be allocated or a higher percentage of the cache that is greater than the default value for the second type of application; and performing least recently used (LRU) based replacement of tracks in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated for the plurality of applications.
 9. The system of claim 8, wherein the second type of application includes at least one of an operating system, a device driver, and a real-time application, and wherein the second type of application is not throttled by demoting tracks of the second type of application prior to expiry of minimum retention time of the tracks of the second type of application, even if the tracks of the second type of application occupy the entirety of the cache.
 10. The system of claim 8, wherein the default value for the first type of application does not exceed 25% of the cache.
 11. The system of claim 8, the operations further comprising: in response to determining that an application has specified a maximum percentage of cache for allocation to application, setting the maximum percentage of cache for allocation to the application to the specified maximum percentage.
 12. The system of claim 8, the operations further comprising: in response to determining that tracks of an application that are stored in the cache exceed the maximum percentage of cache allocated for the application, demoting one or more tracks of the application from the cache even though a minimum retention time in cache has been indicated for the one or more tracks of the application, while performing least recently used (LRU) based replacement of tracks in the cache.
 13. The system of claim 8, wherein the plurality of tracks is a first plurality of tracks, and wherein no indication of a minimum retention time in the cache is received for a second plurality of tracks.
 14. The system of claim 13, the operations further comprising: demoting a track of the first plurality of tracks from the cache, in response to determining that the track is a LRU track in a LRU list of tracks in the cache and the track has been in the cache for a time that exceeds the minimum retention time: and demoting a track of the second plurality of tracks from the cache, in response to determining that the track of the second plurality of tracks is a LRU track in a LRU list.
 15. A computer program product, the computer program product comprising a computer readable storage medium having computer readable program code embodied therewith, the computer readable program code configured to perform operations in a computational device, the operations comprising: receiving, by the computational device, an indication of minimum retention times in a cache for a plurality of tracks of a plurality of applications; in response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, setting the maximum percentage of cache for allocation to the first type of application to a default value; and in response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, allowing an entirety of the cache or a higher percentage of the cache that is greater than the default value to be allocated for the second type of application; and performing least recently used (LRU) based replacement of tracks in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated for the plurality of applications.
 16. The computer program product of claim 15, wherein the second type of application includes at least one of an operating system, a device driver, and a real-time application, and wherein the second type of application is not throttled by demoting tracks of the second type of application prior to expiry of minimum retention time of the tracks of the second type of application, even if the tracks of the second type of application occupy the entirety of the cache.
 17. The computer program product of claim 15, wherein the default value for the first type of application does not exceed 25% of the cache.
 18. The computer program product of claim 15, the operations further comprising: in response to determining that an application has specified a maximum percentage of cache for allocation to application, setting the maximum percentage of cache for allocation to the application to the specified maximum percentage.
 19. The computer program product of claim 15, the operations further comprising: in response to determining that tracks of an application that are stored in the cache exceed the maximum percentage of cache allocated for the application, demoting, by the computational device, one or more tracks of the application from the cache even though a minimum retention time in cache has been indicated for the one or more tracks of the application, while performing least recently used (LRU) based replacement of tracks in the cache.
 20. The computer program product of claim 15, wherein the plurality of tracks is a first plurality of tracks, and wherein no indication of a minimum retention time in the cache is received for a second plurality of tracks.
 21. The computer program product of claim 20, the operations further comprising: demoting a track of the first plurality of tracks from the cache, in response to determining that the track is a LRU track in a LRU list of tracks in the cache and the track has been in the cache for a time that exceeds the minimum retention time; and demoting a track of the second plurality of tracks from the cache, in response to determining that the track of the second plurality of tracks is a LRU track in a LRU list.
 22. A storage controller, comprising: a cache; and a processor coupled to the cache, wherein the processor is configurable to perform operations, the operations comprising: receiving an indication of minimum retention times in the cache for a plurality of tracks of a plurality of applications; in response to determining that a first type of application has not specified a maximum percentage of cache for allocation to the first type of application, setting the maximum percentage of cache for allocation to the first type of application to a default value; and in response to determining that a second type of application has not specified a maximum percentage of cache for allocation to the second type of application, allowing an entirety of the cache or a higher percentage of the cache that is greater than the default value to be allocated for the second type of application; and performing least recently used (LRU) based replacement of tracks in the cache while attempting to satisfy the minimum retention times and the maximum percentage of cache that are allocated for the plurality of applications.
 23. The storage controller of claim 22, wherein the second type of application includes at least one of an operating system, a device driver, and a real-time application, and wherein the second type of application is not throttled by demoting tracks of the second type of application prior to expiry of minimum retention time of the tracks of the second type of application, even if the tracks of the second type of application Occupy the entirety of the cache.
 24. The storage controller of claim 22, wherein the default value for the first type of application does not exceed 25% of the cache.
 25. The storage controller of claim 22, the operations further comprising: in response to determining that an application has specified a maximum percentage of cache for allocation to application, setting the maximum percentage of cache for allocation to the application to the specified maximum percentage. 